Fishbone type array with dipole spacing increasing towards the smaller end



' 1 Fewl, 19

I 69 L H. FINQEBURGHJR. ET'AL 4 FISHBONE TfPE ARRAY WITH DIPOLE SPACING INCREASING Filed April -12, 1968 TOWARDS THE SMALLER END' l w 7 Sheet 4' of 5 EBINgENTORg v 5 5 LEWIS H FINN u EH50 ROBERT QLKRANEK I ATTORNEZS;

' I F'iled Aprii 12 1968 L; H. FINNEBURGH, JR; ETALC 3,427,659 FISHBQNE TYPE ARRAY WITH DIPOLE 'SPACING INCREASING a TOWARDS THE-SMALLER END -Sheet Z OX'IS INVENTORS LEWIS H 'FINNEBURGH,JR.

' ROBERT c. KRANEK Feb. 11; 969 H. FINNEBURGH. JR. ETAL 3 ,65

' 'FISHBQNE TYPE ARRAY WITH DIPOLE SPAGING; INCREASING I v 3 TOWARDS THE SMALLER Filed April 12.. 1968 v L i v Y .IINVENTORS LEWIS H. FmNEBuReHJaj ROBERT c. KRANEKV United States Patent 18 Claims ABSTRACT OF THE DISCLOSURE Broadband radio frequency antennas comprising an endfire array of dipoles which may be either simple dipoles for single mode operation, V-dipoles for dual mode operation, or simple dipoles together with parasitic elements giving the array dual mode operating characteristics. At least four dipoles of varying electrical lengths are spaced apart in file in a substantially coplanar array in order of dipole lengths, the dipole feed, lengths, and spacings being selected to provide endfire operation. The gain of the antennas over the bands to be covered is 0ptimized by employing a dipole spacing that increases progressively from the long dipole end of the array to the short dipole end.

Related application This application is a continuation-in-part of application Ser. No. 464,779, filed June 17, 1965, now abandoned.

Background of the invention Relatively recent refinements of radio and television and extensions of the use thereof for public, governmental, military, and private purposes have created a need for more effective antennas which will give greater and more uniform gain over relatively broad frequency bands. As an example, the expansion of the popularity of color television and F M radio has created a need for relatively broadband, directional antennas having substantially uniform, high gain characteristics over each of the several frequency bands devoted to specific television channels and to FM radio transmission and reception. In the case of television, the low band for channels 2 through 6 covers two continuous ranges of 54 me. to 72 mo. and 76 mo. to 88 me. while the high band for channels 7 through 13 covers another continuous range from 174 me. to 216 mc.; and it is not only highly desirable that a television antenna operate with uniformly high gain over the frequency range of each television channel in both the low and high bands, but that it do so with higher gain over the frequency ranges of the higher frequency channels. Similarly, it is desirable that an antenna for the UHF television band extending from 470 me. to 890 me. be effective over all of that range that is used for normal TV broadcasting, with higher gain at the higher frequencies. In some instances, it is desirable that the same antenna have such performance characteristics over all of the low band, high band, and UHF television frequency ranges.

While endfire antenna arrays comprising a considerable number of simple or complex driven dipoles have long ice been used to provide moderately high gain over one or more relatively broad frequency bands, they have been subject to certain limitations in this regard and have generally not provided optimum gain over such frequency bands. This situation has existed in certain military uses of radio in which it is desired to have a single antenna that is capable of receiving or transmitting with uniformly high efficiency at any frequency over broad frequency bands in which the highest frequency may be from 10 to 20 times the lowest frequency, or even greater.

It was for such very broadband uses that the logperiodic dipole antenna of U.S. Patent No. 3,210,767 of Dwight E. Isbell was developed. The antenna of that Isbell application employed simple, half-wave dipoles of progressively decreasing length and spacing from the rear of the antenna to the front of the antenna. The length of the dipoles decreased by a constant scale factor from approximately a half-wave at the lowest frequency to approximately a half-wave or somewhat less (e.g., wavelength) at the highest frequency of the frequency range to be covered, and the spacing of the dipoles also decreased from the back to the front of the antenna by the same constant scale factor. When both the length and spacing of a suflicient number of dipoles in such an array are so varied according to the same common scale factor, any group of several successive elements should operate with moderately high gain at the mean frequency for which they are dimensioned in essentially the same manner as any other similar group of elements in the array at other frequencies in the range being covered. Thus, such an array should be essentially independent of frequency in its operation and produce substantially the same gain at all frequencies over the entire range.

Substantially concurrently with the work of Isbell on which his U.S. Patent No. 3,210,767 was based, Arthur F. Wickersham developed the log periodic dipole antennas of his U.S. Patent No. 3,056,960. The antennas of the Isbell patent were conventionally driven from the small dipole end by a pair of feeder conductors that extended along the axis of the array and were electrically connected to the dipoles in succession with transposition of the feeder conductors between their connections to adjacent dipoles to provide unidirectional endfire operation. The antennas of the Wickersham patent were also driven from the small dipole end by a pair of feeder conductors that extended along the axis of the array and were electromagnetically coupled to the dipoles in succession without transposition of the feeder conductors between adjacent dipoles, the dipoles being referred to as shunt excited by such coupling to the feeder conductors. Here, also, the result was unidirectional endfire operation like that of the antennas of the Isbell patent.

As has also long been known and as disclosed in U.S. Patent No. 2,817,085 to Schwartz and Lo, similar endfire arrays of dipoles may be driven from the large dipole end of the array by a pair of feeder conductors that extend along the axis of the array and are electrically connected to the dipoles in succession without transposition of the feeder conductors between adjacent dipoles. In this case, the driven array as a whole may be bidirectional to some degree along the axis of the array with the direction of greatest effectiveness when operating in the fundamental mode being dependent upon the phase relationship of the driven dipoles, which, in turn, is dependent upon the magnitude of the driven element spacing and the lengths of the driven elements relative to the wavelengths of the radiation over the frequency range for which the antenna is used. The array of driven elements may be made more nearly unidirectional (if desired) by the use of a conventional reflector and with the optional use of one or more directors as well.

All such feed systems are generally applicable to multiple dipole arrays for achieving endfire operation. Those skilled in the art are familiar with such feed systems and how to select an appropriate number of elements and appropriate element lengths and spacings for use with a particular feed system over a given frequency range with the desired directional operating characteristics.

The ability to cover a range of frequencies much greater than 2:1 (referring to the ratio of maximum to minimum frequency) with the antenna feed system of the above-mentioned Isbell patent and while employing only fundamental mode operation requires not only a decreasing dipole length from back to front of the array but, also, a decreasing spacing of the dipoles from back to front of the array. Only in that way can the spacing of any two adjacent elements relative to their average length always be kept substantially below a half wavelength at their mean frequency of operation. Otherwise, Where such spacing closely approaches one-half of the mean operating wavelength, the dipoles are connected in phase at that frequency and will no longer operate as an endfire array with a direction of greatest effectiveness toward the front. Instead, in that case, the antenna will operate as a broadside array with a direction of greatest effectiveness that is at right angles to the desired direction of transmission or reception, i.e., generally vertically.

Where smaller frequency ranges below about 2:1 are to be covered, the lengths of the rearmost and forwardmost halfwave dipoles also differ by a ratio of only about 2:1 (slightly greater than the frequency ratio), and it is feasible to use a constant spacing of the dipoles from end to end of the array with the feed system of the Isbell patent, provided only that such spacing is distinctly less than one-half wavelength at the highest operating frequency. Uniform spacing has sometimes been used merely for simplicity of design and manufacture, though it involves distinct departures from any known theory for the optimum spacing of any two adjacent dipoles in such an endfire array for maximum gain reinforcement at all frequencies.

In the case of the feed system of the above-mentioned Schwartz and Lo patent, by contrast, an array of dipoles of equal length and no transposition of the feeder conductors between dipoles has bidirectional endfire characteristics with a front-to-back ratio that can be near unity when the dipoles are all spaced a half wave apart at their common half-wave resonant frequency and impedances are balanced to maintain the same current in all dipoles. Both above and below a half wavelength spacing, endfire operation also occurs with a front-toback ratio that varies with the magnitude of the spacing and the current distribution among the dipoles. When the dipoles are varied in length progressively from one end of the array to the other for better covering a substantial range of frequencies, as in the Schwartz and Lo patent, the most desirable dipole spacing will generally be found to be substantially less than a half wavelength for the shortest dipoles at the upper end of the frequency band to be accommodated. In determining the optimum spacing for a given array of dipoles, of course, additional factors should be taken into consideration, such as the problem of impedance matching and balancing and limitations on the physical size of the antenna that may be dictated both by the use to be made of the antenna and the cost of manufacturing and installing it.

Regardless of the type of feed system used for multiple dipole endfire arrays, uniform spacing of the dipoles from end to end of the arrays, as well as the log periodic scaling of the above-mentioned Isbell and Mayes et al. patents have been used or proposed heretofore. The latter scaling is essential to operation with frequency independent periodicity; and a spacing that decreases in the direction of decreasing dipole length is required by generally accepted theory if anything approaching frequency independent operation is desired. In some instances, discontinuous dipole spacing patterns (i.e., departures from uniform spacing or from a uniform decrease in spacing with uniform decreasing dipole lengths) have been used to compensate for various performance abnormalities, as dictated by empirical testing and adjustment or by a desire to operate over spaced portions of a broad frequency band. U.S. Patent No. 3,150,376 to Carrel et al. discloses one example of a discontinuous dipole spacing pattern.

Thus, although uniform spacing of the dipoles has been used from time to time as a matter of convenience, optimum spacing for substantially uniform gain obviously dictates reducing the spacing progressively from the back to the front of the array and at least generally in accordance with a reduction in dipole lengths, whether or not the dipole lengths are varied by a constant scale factor for the purposes of the above-mentioned Isbell and Mayes et al. patents.

Summary of the invention The present invention involves generally coplanar, endfire arrays of dipoles. The invention is based upon an accidental discovery made in the course of experimental efforts to eliminate certain operational deficiencies of prior, commercial, multiple dipole, endfire antennas. In accordance with that discovery, a radical departure from conventional design theory as to the ideal dipole spacing has been found to produce a wholly unexpected improvement in the operation of such antennas for many common service applications where each continuous frequency band to be covered is of only moderate width, e.g., involving a ratio of only about 2:1 or less between the maximum and minimum frequencies. As will be more fully explained hereinafter, the improvement resulting from this discovery is applicable for improving the gain of generally coplanar endfire arrays of simple dipoles operating in a single mode at or near their half-wave resonant frequencies and, also, for improving the gain in higher mode operation of endfire arrays of dipoles that have been modified to provide such operation. As disclosed in the above-mentioned Schwartz et al. Patent No. 2,817,085 and Greenberg Patents Nos. 3,086,206 and 3,163,864, and in Patents No. 3,108,280 (Re. 25,740) to Mayes et al. and No. 3,150,376 to Carrel et al., such dual mode operation results merely from modifying the dipoles to render them individually operative in two or more modes.

The improved gain of endfire antenna arrays involving a multiplicity of generally coplanar driven dipoles of varying electrical lengths that decrease toward the front of the arrays is achieved in accordance with the present invention by employing a spacing of the dipoles that decreases continuously from the short to the long dipole end of the arrays. Such spacing is to be contrasted primarily with the uniform dipole spacing in the abovementioned Greenberg Patent No. 3,163,864, and the dipole spacing that decreases from the long to the short dipole end of the arrays in the above-mentioned Isbell Patent No. 3,210,767, Mayes et al. Patent No. 3,108,280 (Re. 25,740), and Carrel et al. Patent No. 3,150,376. In a large number of comparative tests, using different arrays with different numbers and types of dipoles, distinct and substantial gain increases have resulted when the dipole spacing has been changed from those prior art spacings to the spacings on which the present invention is based.

Although the antennas of the present invention are referred to herein as being generally coplanar, substantial departures from strictly coplanar dipole arrangements are permissible without materially affecting endfire operation or the benefits of the present invention. Such departures, which may be as great as about 15-20% or more of the minimum operative wavelength of the array, have been frequently employed in practice for simplifying the balancing of the impedances and the feeding of the dipoles, and also for simplifying and rigidifying the physical structure of the antenna.

Description of drawings In the accompanying drawings:

FIGURE 1 is a perspective view of one form of dual mode antenna array embodyin the present invention;

FIG. 2 is a plan view of the embodiment of the invention shown in FIG. 1;

FIG. 3 is a schematic electrical diagram of an antenna of the present invention that may be physically constructed in accordance with FIGS. 1 and 2 and that is made up of seven active V-dipoles for dual band operation;

FIG. 4 is a schematic electrical diagram similar to FIG. 3, but showing an antenna of the invention made up of five active, straight dipoles, each having a relatively much shorter parasitic element closely spaced in front thereof to render it active on substantially the third higher harmonic of its half-wave resonant frequency; and

FIG. 5 is a schematic electrical diagram similar to FIGS. 3 and 4, but showing an antenna made up of six active, straight dipoles, only some of which are provided with closely spaced, relatively much shorter, parasitic elements in front thereof for rendering them active on their respective third higher harmonic frequencies.

FIG. 6 is a perspective view of a single mode, simple dipole array embodying the present invention;

FIGS. 7 and 8 are fragmentary views of the antenna of FIG. 6 showing details of its construction, FIG. 8 being a sectional view taken as indicated by the line 88 in FIG. 7; and

FIG. 9 is a schematic electrical and dimensional diagram of the antenna of FIGS. 6-8.

Detailed description of invention Referring first to FIGS. 1 and 2, the basic electrical elements of an antenna 10 are shown mounted on a horizontally extending boom 12 that is in turn mounted on a vertical mast 13 by means of a U-bolt 14. The antenna 10 comprises a series of conventional V-dipoles 17 of varying physical and electrical lengths that lie substantially in a common horizontal plane and are spaced apart in file with the opposite arms of each dipole oriented substantially parallel to the respectively corresponding arms of the other dipoles. As shown in FIG. 2, each dipole extends transversely to opposite sides of a longitudinal axis of symmetry of the array that extends generally along the length of the boom 12. As indicated by reference characters on the longest dipole at one end of the array, each of the dipoles has its oppositely extending arms 17a and 17b mounted on an insulator 18 which, in turn, is mounted on the boom 12, the inner ends of the oppositely extending arms being spaced apart to form a center gap at the apex of the V-configuration of each dipole.

The V-dipoles 17 are interconnected for operation in an endfire manner by means of a pair of feeder conductors 21 which, in this instance, are crossed so that their respective connections to each dipole are transposed relative to their connection to the next dipole in either direction along the boom. With this transposition of the connections of the feeders to successive dipoles, the conventional, two-conductor transmission line 22 is connected to terminals T and T at the front end of the array, which is the end at which the smallest dipole is located, so that the antenna, if operated as a transmitting antenna, would have its maximum radiation in the direction of the arrow 23. When the antenna is operated as a receiving antenna, for which this embodiment of the invention was particularly designed, the arrow 23 indicates the proper orientation of the antenna with respect to a transmitting station toward which the arrow points.

The antenna of FIGS. 1 and 2 operates over frequency bands that are primarily determined by the lengths of the successive v dipoles and, in part, by their relative spacing one from another and by the included angle a (FIG. 2) between the dipole arms. Commonly, the length of a V- dipole is considered to be the sum of the actual lengths of the two arms plus the width of the center gap between them, i.e., the tip-to-tip length of the dipole when the arms are swung into collinear alignment. In the ensuing discussion of antennas of the type shown in FIGS. 1 and 2, the lengths of the dipoles are considered in that manner. Since the oppositely extending arms of each dipole are parallel to the corresponding arms of the other dipoles, the spacing between dipoles may be measured at any point in a direction parallel to the axis of symmetry of the array.

As previously indicated, V-dipole antennas of the type shown in FIGS. 1 and 2 respond substantially as an end fire array of straight, half-wave dipoles of the same tip-totip lengths at frequencies within the lower of the two frequency bands for which they are designed. In a higher frequency band which is roughly three times the frequencies of the lower band, such antennas respond substantially as 3/2-wave dipoles, but with the three halfwave current loops in each dipole being fed to the feeders nearly in phase, provided the center angle a is properly selected according to known principles for third harmonic operation. Although still higher frequency bands roughly corresponding to higher odd harmonics of the lower frequency band can also be covered with such antennas, a different center angle a is required for optimum performance in each higher harmonic range and can only be used with a sacrifice in performance in other harmonic ranges. Also, the above-described spacing limitations become even more critical at higher harmonic frequencies. Accordingly, for the purposes of the present invention, only operation of the antennas in the half-wave mode and 3/2-wave mode are considered.

When designing antennas of the type shown in FIGS. 1 and 2 according to the log-periodic theory of the abovementioned Mayes et a1. Patent Re. 25,740, the length of each dipole and its spacing from adjacent dipoles on either side thereof both vary along the array precisely in accordance with the wavelength for which each dipole is intended to operate as a half-wave dipole. When designing other conventional endfire arrays, the individual dipole lengths have commonly been selected to correspond approximately to half wavelengths at approximately equal frequency intervals along the band to be covered or intervals determined primarily by specific frequencies or specific, narrow ranges of frequencies for which the antenna is desired to be operative with optimum response. In both of these cases, the spacings between dipoles were desirably made as small as practical without producing adverse effects, in order to keep the total length of the array to a minimum for structural and economic reasons. Any continuous progressive change in spacing along the array has involved a decreasing spacing toward the end of the array containing the shorter elements, since the optimum spacing of dipoles in an endfire array has been assumed to be related to the wavelengths for which the individual elements are designed, and these lengths decrease from the back to the front of the array.

The present invention is based upon an initial discovery that distinct improvements in the gain over portions of the bands to be covered with a dual band endfire array of the type shown in FIGS. 1 and 2 can be obtained by employing dipole spacings that increase (rather than remain constant or decrease) from the back to the front of the array, all other factors, including overall length of the array along the boom, being the same. Because such spacings of the dipoles, if continued for covering a very wide frequency band, would reach an inoperative value of a half wavelength when using the feed system of this embodiment of the invention, it is not practical, in this case, to cover a bandwidth much greater than 2:1 with fundamental mode operation. Moreover, boom length considerations may similarly limit the bandwidths which it is practical to cover. Also, when the dipole spacing approaches certain values much above a half wavelength, various radiation pattern aberrations are encountered. Thus, the present invention is directed to covering only moderately broad bandwidths not greatly exceeding a 2:1 frequency range.

In prior endfire arrays of the type shown in FIGS. 1 and 2, the higher of the two frequency bands that will be covered effectively is not precisely a third harmonic of the lower band. Thus, when an antenna of the type shown in FIGS. 1 and 2 is designed with log periodic dipole lengths and spacings to cover a low frequency range of about 54 mc. to about 88 me. (low VHF television band), optimum operation in the next higher mode actually begins at and extends upwardly from around 185- 190 mc. (about 3.5 times the frequency at the low end of the low band). As a result, such antennas have often failed to provide satisfactory response on channels 7 and 8 and to some degree on channel 9 of the high VHF television band. When such an antenna is designed with uniform spacing instead of log periodic spacing of the several dipoles, the same kind of performance deficiency occurs, though to a somewhat less pronounced degree. While this disadvantage could be overcome by adding more and longer dipoles at the rear end of the array, this is no solution commercially where the objective is to get maximum performance from a given number of elements and minimum length of boom. The present invention has also alleviated this problem.

The manner in which the present invention overcomes the deficiencies of prior art antennas of the type discussed above, while also improving the gain over additional portions of both the high and low frequency bands, will now be explained and illustrated with reference to FIG. 3 of the drawings showing a seven-element, V-dipole array of the character depicted in FIGS. 1 and 2, and with refer ence to tables of dipole length and spacing relationships and performance data for the antenna of FIG. 3 and for a similar antenna having uniform dipole spacing as taught in the above-mentioned Greenberg patents.

Referring to FIG. 3, the several dipoles E E E and the interconnecting feeder conductors 21 and transmission line terminals T and T are shown diagrammatically with the differing spacings S between dipoles indicated by the dimensions S S S As a matter of convenience, the lengths L of the dipoles are indicated on the diagram only for the longest and shortest dipoles by the dimensions of L and L In designing a seven-element V-dipole antenna according to FIG. 3, element lengths graduated uniformly from a maximum dipole length L, of about 103 to a minimum dipole length L; of about 45" were initially selected and were uniformly spaced apart a distance of 13" from end to end of the array. The angle a was 120 for all of the Vdipoles. This initial design was then placed on a test range and numerous adjustments in dipole lengths were empirically arrived at to obtain optimum dipole lengths for maximum gain over each of the low VHF and high VHF television channels. The resulting length and spacing relationships are shown by the first three columns of Table IA. Then, without changing the lengths of any of the dipoles, the spacings were changed to the figures shown in the fourth column of Table IA. The antenna was retested in the same manner on the same test range. As a result of this further departure from the initial design, the gain of the antenna was improved materially over most of the low band VHF television channels and high band VHF television channels. However, by making additional empirical adjustments in dipole lengths as were apparently required by the effects of altering the dipole spacing, the

gain over the low band and high band VHF television channels was still further improved. The final maximum gain resulting from altering both the lengths and spacings of the dipoles are shown in Table IB in terms of db. As will be noted, the improvement was particularly great on channel 7 and was substantial on channels 2, 3, 6 and 8. The gain over the antenna with the original uniform spacing ranged down to zero db for some channels or for specific frequencies, as shown in Table IE, but in no case was the gain reduced by the altered dipole lengths and spacings.

Since, for practically all types of dual band antennas of the prior art designed to cover the low and high band VHF television frequencies, channels 7 and 8 have caused the greatest problems for a variety of reasons, the ability of the present invention to improve the gain on these two channels to such a marked degree is particularly significant. When it is understood that a gain of 3 db represents a 100% increase in the power or energy received from an antenna, the significance of the improvements shown in Table IE will he better appreciated.

TABLE IA.7-ELEMENT V-DIPOLE ANTENNA Least. age are S, Inches L, Inches S, Inches 93 13 96 12% 90 13 12% 79 13 78 12% E 73 13 72 13% E0 65 13 64 13% E 45 13 44 13% Total Length 78 78 TABLE IB.GAIN RESULTING FROM ALTERED LENGTHS AND SPACINGS TV Channel Test Freq., mc. Gain, db

meow N Band between 88 and 174 me. not used by TV ooocooocooppppp nwozou:

As a further illustration of the benefits of the present invention, the same comparison is made in Tables IIA and HE between two 9-element V-dipole arrays of the same physical design shown in FIGS. 1 and 2 except for diiferences in length and spacing of the dipoles and the addition to each of the 9-element versions of an identical high band director identically spaced in front of the foremost active dipole and dimensioned for channel 13. As shown in Table IIB, a gain improvement was realized on fewer channels in the low band in this case, but was slightly greater on channels 2 and 7, for which the single TABLE IIA.9-ELEMENT V-DIPOLE ANTENNA WITH 1-HIGH BAND V-DIREOTOR Length Original Altered Altered Dipole L, Inches Spacing Length Spacing S, Inches L, Inches S, Inches Total Length 104 104 TABLE IIB.GAIN RESULTING FROM ALTERED LENGTHS AND SPACINGS TV Channel Test Freq., Inc. Gain, db

Band between 72 and 76 me. not used by TV Band between 88 and 174 me. not used by TV Similar advantages from the invention are obtained when multiple director and/ or deflector arrays are added to antennas of the types compared in the foregoing tables.

Referring now to FIGS. 4 and 5, these figures diagrammatically show antennas embodying the present invention with straight active dipoles substituted for the V-dipoles of FIGS. 1 and 2. In order to obtain satisfactory dual band operation, a number of short parasitic elements are closely spaced in front of a corresponding number of the straight active dipoles.

FIG. 4 shows a '5-active dipole array connected in an endfire manner, each of the dipoles E E E being conventional, straight, half-wave dipoles, and each having a short parasitic element P P or P closely spaced in front thereof. As disclosed and claimed in Patent Re. 24,413 of Robert S. Weiss, these short parasitic elements are approximately one-third the length of the half-wave active dipoles with which they are respectively associated and, hence, are substantially a half wavelength at frequencies which are third harmonics of the fundamental half-wave frequencies of the active dipoles. By spacing such parasitic elements from their respectively associated straight active dipoles by a distance of about 1% to about 7% of the half wavelengths for which the straight active dipoles are respectively designed, the combination of straight dipoles and closely spaced associated parasitic elements gives each of the straight active dipoles dual band characteristics closely similar to the dual band characteristics of active V-dipoles corresponding in length to the active straight dipoles. Thus, the antenna of FIG. 4 also embodies the present invention and has operating characteristics basically the same as those of the antenna of FIGS. 1 and 2.

FIG. 5 shows a modified version of the antenna of FIG. 4 involving one more active dipole E and a single, high band director D disposed in front thereof, but with parasitic elements P P and P respectively closely spaced in front of only the active straight dipoles E E and E In many instances, it will be found that no advantage in the higher of the two frequency bands for which an antenna of this type is designed results from including such short, closely spaced parasitic elements in front of each of the straight active dipoles and that, therefore, the same performance can be achieved with a lesser number of elements by eliminating some of those closely spaced parasitic elements. This seems to be particularly true when one or more directors, such as the director D, is used in conjunction with the endfire array as shown in FIG. 5.

In both of the antennas of FIGS. 4 and 5, it will be noted from the drawing that the straight active dipoles decrease in length from the back toward the front and that the spacing of successive, active, straight dipoles increases continuously from the back toward the front as indicated in both figures by the visible progressive increase in dimensions S S etc. As in the other embodiments of the present invention illustrated in FIGS. l-3

and in the foregoing tables representing several variants thereof, the optimum active dipole lengths and spacings are best determined empirically, but can be generally similar to the dimensional relationships set forth in the foregoing tables.

As more recently discovered, the advantages of the invention are also obtainedto a pronounced degree in end fire arrays of simple dipoles operating only on a single mode and connected one to another by a pair of parallel feeder conductors without phase transposition between dipoles. Such an antenna is illustrated in FIGS. 6-9.

Referring to FIGS. 6-8, the basic electrical elements of the antenna 30 are shown mounted on a horizontally extending boom 32 that is in turn mounted on a vertical mast 33 by means of a U-bolt 34. The antenna 30 com prises a series of 10 simple dipoles 37 of varying physical and electrical lengths that lie substantially in a common horizontal plane and that are spaced apart in file with the opposite arms of each dipole arranged in collinear alignment and parallel to the arms of the other dipoles. Each dipole extends transversely to opposite sides of a longitudinal axis of symmetry of the array that extends generally along the length of the boom 32.

As indicated by reference characters on the longest dipole at one end of the array, each of the dipoles has its oppositely extending arms 37a and 37!; mounted on an insulator 38 which, in turn, is mounted on the boom 32, the inner ends of the oppositely extending arms being spaced apart to form a center feed gap.

The dipoles 37 are interconnected for operation in an endfire manner by means of a pair of feeder conductors 41 which, in this instance, are parallel throughout the length of the array and are connected at the long dipole end of the array to terminals T and T When driven in this manner, without phase transposition of the feed between successive dipoles, the impedance relationships and dipole spacings are such that the antenna is highly unidirectional and operates for receiving from a transmitting station located in the direction of the arrow 43.

It is contemplated that a reflector (not shown) may be employed beyond the longest dipole to give the antenna an even better front-to-back ratio. It is also contemplated that one or more directors (not shown) may be employed beyond the shortest dipole to raise the forward gain at particular frequencies.

As shown on a larger scale in FIGS. 7 and 8, the dipole arms 37a and 37b of the illustrated antenna are shallow channel-shaped strips, suitably made of sheet aluminum or the like. To permit folding of the dipole arms for convenient packaging of the antenna, the dipole arms are preferably mounted on the insulator 38 by respective rivets 42a and 42b. For anchoring the dipole arms in their extended, operative positions, the inner ends thereof are notched at 43 to snap over and receive insulator protuberances or ribs 44. Curved, transverse grooves in the upper surface of the insulator closely adjacent the rivets 42a and 42b receive correspondingly curved portions 46 of the feeder conductors 41, each dipole arm engaging its associated feeder conductor and being held firmly in contact therewith by its mounting rivet. Each insulator is also provided with a central, transverse notch in its lower surface that receives the antenna boom 32, to which the insulator is secured by a central rivet 48 so as to prevent relative turning of the insulator and boom about the central rivet.

The particular antenna illustrated in FIGS. 69, when dimensioned for operation over the portion of the UHF television band from 470 to 806 me. that has been assigned to regular broadcasting channels 14 through 69 (excluding the upper portion of the UHF band reserved for the 14 translater channels 70 through 83), has been constructed with element lengths L as tabulated in Table IIIA, in which the individual dipoles are designated E through E as also shown in FIG. 9. Two versions of the antenna were constructed for comparison purposes, one version having the dipoles uniformly spaced over a boom length of 29" with a center-to-center dipole spacing of 3%" and the other version having the dipole spacing progressively increase from the rear (longest dipole end) to the front (shortest dipole end) with equal increments of change from one space to the next. The dipole spacings of the two versions are also tabulated in Table IHA and are respectively designated Original Spacing-S and Altered Spacing-S.

Although the antenna of FIGS. 6'-9 is highly effective over the UHF frequency range of 470 through 840 mc., it will be noted from Table IIIA that the longest dipole having a tip-to-tip length of 14%" is substantially longer than a half-wave at 470 me. (about 12 /2") and that the shortest dipole having a tip-to-tip length of 10 /4" is substantially longer than a half-Wave at 840 mc. (about 7"). These dipole lengths Were arrived at empirically for obtaining optimum gain and endfire radiation patterns over the range of 470 me. to 806 mc. with the particular antenna consrtuction and feed shown in FIGS. 6-9. Surprisingly, the forward lobe of the radiation patterns over the entire UHF frequency range of 470 to 890' mc. is much narrower than is normal for half-Wave mode operation of an endfire array. Why such abnormally long dipoles are required to provide optimum gain and radiation patterns over that frequency range has not yet been determined with certainty. The explanation may involve, as the principal consideration, obtaining a more favorable average match between the impedance of the antenna as a whole and the impedance of the 300 ohm transmission line that is standard for television antennas and was used in all of the tests described herein.

The two versions of the antenna were successively placed on the same test range, and the relative gain resulting from altering the spacing in accordance with the present invention was determined at 19 test frequencies. The results are tabulated in Table I IIB. As will be observed from the latter table, the second version employing dipole spacings according to the present invention (4th column of Table IIIA) resulted in a significant positive gain at 17 of the 19 test frequencies and a negative gain at only two test frequencies. As will also be noted, the positive gain ranged from 1.0 db to as high as 5.6 db at 13 of the 19 test frequencies, and the negative gain was as great as 1.0 db on only one test frequency and was only 0.2 db on the other test frequency where negative gains occurred.

As will be appreciated by those skilled in the art, the negative gain of the second version at 780 and 800 mc. could be substantially or completely eliminated by small empirical adjustments of element lengths or spacings, or both, toward the small dipole end of the array without materially atfecting the positive gain on other frequencies. Also, where parasitic elements are employed, they may be empirically selected and adjusted to bring about a better balance of the inherent gain of the driven dipole array over the entire frequency band sought to be covered, or to enhance the performance in a portion of the band of greatest importance with minimal deterioration of the performance in other, less important portions of the band. Both of these expedients have been successfully employed heretofore in commercial TV antennas employing a multiplicity of dipoles in endfire arrays.

A striking aspect of the advantages of the present invention is that the gain improvements are achieved with the same number and kind of antenna elements made from the same quantity of structural material (or slightly less) and arranged in arrays of the same size as in prior art designs providing inferior performance. Thus, with no basic design or manufacturing change in other respects, the disclosed improvement is achieved merely by redistributing the same active elements along the length of the same boom, at no increase in cost or antenna size.

TABLE IIIA.10-ELEMENT SIMPLE DIPOLE ANTENNA Length L, Original Altered Dipole Inches Spacing Spacing S, Inches S, Inches 3% 2 MB E 13% 3% 3 En a 10% Table IIIB.Gain resulting from altered spacings Gain from altered 13 Table IIIB.--Continued Test freq., mc.: Gain from altered spacing As should be apparent, antennas may be designed in accordance with the present invention using any desired number of active dipoles, from a minimum of four up to any desired number within reason, for covering a desired frequency band over which the frequencies vary within a ratio of not greatly exceeding 2:1 and, where desired, for also covering an additional, higher frequency band bearing substantially a third harmonic relationship to the lower frequency band.

Considering the magnitude of the incremental changes in element lengths along the array of antennas of the present invention, it should be obvious that such incremental changes will normally become smaller as the number of dipoles employed to cover a given frequency range increases. This is indicated by a comparison of the incremental changes in dipole lengths shown in Tables IA and IIA. Whatever this incremental change may be, as governed by the width of the frequency bands to be covered and by the number of dipoles employed, the ratio of the maximum to minimum lengths of the dipoles should approximate or, preferably, somewhat exceed the ratio of maximum to minimum wavelengths in the widest frequency band to be covered. As previously indicated, the latter ratio will generally not greatly exceed 2:1.

Considering the magnitude of the incremental changes in dipole spacings along the array, these may be quite small while having marked effects upon the performance of an antenna, as clearly indicated by the data in the foregoing tables in which the spacings of the dipoles increase from space to space by a factor ranging from as low as about 1.02 (Tables IA and HA) to a maximum of only about 1:12 in the several embodiments of the invention disclosed herein. Also, the incremental spacing changes need not be uniform and may advantageously vary in some instances. A variety of considerations may so effect the optimum average spacing and average incremental increase in spacing as to make it difiicult to ascertain any generally applicable limit for the maximum average incremental increase.

While the present invention has been illustrated and described herein with reference to a number of specific antenna structures and dimensional relationships of the dipoles therein, it will be appreciated by those skilled in the art that the invention is not limited to any of those precise physical structures or dimensional relationships. Accordingly, the invention is considered to include such variants as are embraced within the scope of the appended claims.

What is claimed is:

1. A directional antenna for operation over a predetermined frequency band, said antenna comprising a group of at least four consecutive, generally parallel, driven dipoles of varying electrical length spaced apart in file in a generally coplanar array, each dipole extending transversely to opposite sides of a longitudinal axis of the array; feeder means extending along said axis for driving said dipoles as an endfire array with greatest effectiveness along said axis; the lengths of successive dipoles decreasing along said axis from one end of the array to the other, and the successive spacings of said dipoles increasing along said axis in the direction of decreasing dipole lengths.

2. An antenna according to claim 1, including means for rendering said array operative over a second, higher frequency band that bears substantially a harmonic relationship to said predetermined frequency band.

3. An antenna according to claim 1 in which said feeder means comprise a pair of electrical conductors extending generally along said axis from one to another of said dipoles in succession.

4. An antenna according to claim 1 in which the length of the shortest of said dipoles substantially exceeds the maximum dipole spacing.

5. An antenna according to claim 4 in which the spacing of the two shortest dipoles is less than the length of the shortest dipole and said feeder means connect said dipoles with phase transposition between each pair of adjacent dipoles.

6. An antenna according to claim 1 in which said feeder means comprise a pair of electrical conductors extending generally along said axis from one to another of said dipoles in succession and with direct electrical connections thereto.

7. An antenna according to claim 1 in which said feeder means comprise a pair of electrical conductors extending generally along said axis from one to another of said dipoles in succession and with direct electrical connections thereto, said connections to said dipoles being reversed from one dipole to the next throughout the array so that the feeder connections between successive dipoles are electrically transposed, said antenna including means for conmeeting a two conductor transmission line to said feeder conductors adjacent the shortest of said dipoles.

8. An antenna according to claim 7 in which the spacing of the two shortest dipoles is less than a half wavelength it the highest frequency of said predetermined frequency and.

9. An antenna according to claim 7 in which the spacing of the two shortest dipoles is less than a half wavelength at the half-wave resonant frequency of the shortest dipole.

10. An antenna according to claim 1 in which said feeder means comprise a pair of electrical conductors extending generally along said axis from one to another of said dipoles in succession and with direct electrical connections thereto, one of said feeder conductors being connected to each of said dipoles on one side of the electrical center thereof and the other of said feeder conductors being connected to each of said dipoles on the opposite side of the electrical center thereof, said antenna including means for connecting a two conductor transmission line to said feeder conductors adjacent the longest of said dipoles.

11. An antenna according to claim 6, including means for rendering said array operative over a second, higher frequency band thatbears substantially a harmonic relationship to said predetermined frequency band.

12. An antenna according to claim 6 in which the feeder conductors are electrically transposed between each pair of adjacent dipoles and said antenna includes means for rendering said array operative over a second, higher frequency band that bears substantially a harmonic relationship to said predetermined frequency band.

13. An antenna according to claim 12 in which said means for rendering said array operative over said higher frequency band comprise a plurality of parasitic means respectively associated with a corresponding plurality of said dipoles.

14. An antenna according to claim 7 including means for rendering said array operative over a second, higher frequency band that bears substantially a harmonic relationship to said predetermined frequency band and the spacing between the two shortest dipoles is less than the length of the shortest dipole.

15. An antenna according to claim 7 in which said dipoles are V-dipoles and the spacing between the two shortest dipoles is less than the length of the shortest dipole.

16. An antenna according to claim 12 in which said means for rendering said array operative over said higher frequency band comprise parasitic elements, one such parasitic element being associated with each of said dipoles.

17. An antenna according to claim 1 in which said dipoles are V-dipoles for rendering said array operative over a second, higher frequency band that bears substantially 15 16 a harmonic relationship to said predetermined frequency References Cited band. Saalbach: Antenna Disclosure, by H. K. Saalbach, single 18. An antenna according to claim 7 in which said dih et, May 27, 1963, poles are V-dipoles for rendering said array operative over a second, higher frequency band that bears substantially a 5 ELI'LIEBERMAN Primary Examinerharmonic relationship to said predetermined frequency U.S. C1. X.R. b d, 343-811, 815 

